This invention relates to a liquid crystal display apparatus and a method for driving the same, and more particularly to an active matrix type liquid crystal display apparatus using a liquid crystal material having spontaneous polarization induced by application of an electric field or inherent spontaneous polarization and a method for driving the same.
A liquid crystal display apparatus has features of low power consumption and light weight and is widely used as a display device for a word processor, personal computer, car navigation system, and the like. As the above liquid crystal display apparatus, a TFT-TN system using nematic liquid crystal and using active elements such as thin film transistors (TFTs) as switching elements and an STN system using nematic liquid crystal and having an increased twist angle are known at present, and full-color display type liquid crystal display apparatuses of the above systems with the screen size of approximately 10 inches are already realized and are used as information terminal display devices.
The above liquid crystal display apparatuses have satisfactory characteristics for the limited application such as word processing and calculations on table. However, the STN system is still insufficient in the response speed in the above applications. Further, the viewing angle is extremely small and various studies are now made to enlarge the viewing angle by use of a retardation film, but a sufficiently large viewing angle is not yet attained.
On the other hand, the TN type liquid crystal display apparatus using TFTs as switching elements is substantially satisfied in the response speed, but it is estimated that some difficulty relating to the response speed will occur when a still larger liquid crystal display apparatus is manufactured. In addition, in the TN system, the viewing angle which is larger than that in the STN system can be attained, but the viewing angle becomes extremely small particularly in the case of full-color display and it is considered that the above problem limits the application of the above display system.
As a display system for solving the above problem of the liquid crystal display apparatus, recently, much attention is given to display systems using liquid crystal materials having spontaneous polarization induced by application of an electric field or inherent spontaneous polarization such as ferroelectric liquid crystal FLC, antiferroelectric liquid crystal AFLC, distorted helical ferroelectric liquid crystal DHF, twisted ferroelectric liquid crystal TFLC, or thresholdless antiferroelectric liquid crystal TLAF.
As the display system using the above liquid crystal material, a system using surface stabilized ferroelectric liquid crystal (SSFLC, N. A. Clark and S. T. Lagerewall Appl. Phys. Lett., 36,899 (1980)) is known. According to this system, the response speed is enhanced by two to three figures and the viewing angle is increased to a value equivalent to that of the cathode ray tube.
In the above display system, the switching operation is effected by relieving the helical structure of chiral smectic C phase of smectic liquid crystal by use of the interaction between an alignment film and the liquid crystal and using the torque generated by the interaction between the electric field and the spontaneous polarization caused at this time. In this system, since only two states in which the spontaneous polarization is set in two directions perpendicular to the interface of the alignment film are stabilized, it has a memory performance. Therefore, the system was first greatly expected as a display system in which switching elements constructed by non-linear active elements such as TFTs, TFDs (thin film diodes) or MIMs (metal-insulator-metal diodes) were not required.
However, in the above system, since only the two states are used, a gray scale cannot be displayed. When taking displays used in the future into consideration, display of the gray scale is indispensable and some studies for providing the display of gray scale have been made.
As an example of the above studies, some attempts for displaying the gray scale by using the surface stabilized ferroelectric liquid crystal described above have been made (for example, W. J. A. M. Harmann, Ferroelectrics, 1991, 122, p1). However, the surface stabilized ferroelectric liquid crystal displays a discontinuous switching characteristic called domain inversion in its response and it is substantially impossible to display the gray scale without using active elements.
On the other hand, a display system (A. D. L. Chandani, T. Hagiwara, T. Suzuki, Y. Ouchi, H. Takezoe and A. Fukuda, Jpn. J. Appl. Phys., 271,729 (1988)) using antiferroelectric liquid crystal and utilizing the antiferroelectric liquid crystal phase (SmCa phase) thereof is known. In this system, an antiferroelectric liquid crystal structure is provided at the time of no application voltage in addition to the two stabilized states of the ferroelectric liquid crystal, and recently, it is published that the gray scale display can be attained by use of the above display system without using switching elements constructed by active elements (N. Koshoubu, K. Mori, K. Nakamura, Y. Yamada, Ferroelectrics, 1993, 149, p295).
In addition to the above display systems, recently, display apparatuses using switching elements constructed by active elements and utilizing the chiral smectic C phase thereof are proposed. More specifically, a system using DHF (J. Funfschilling and M. Schadt, J. Appl. Phys. 66(8), 15), a system using TFLC (J. S. Pate, Appl. Phys. Lett. 60(3), p280) and TLAF (Thresholdless Antiferroelectric Liquid Crystal) have been proposed. Since a display apparatus using one of the above systems uses switching elements constructed by active elements, the cost thereof tends to become higher than that of the former systems.
The above systems are superior to the former systems in the following points. First, the reliability of gray scale display is excellent. That is, in the above systems, a variation in the transmittance with respect to an application voltage is relatively smooth and a problem that the gray scale display becomes difficult as in the surface stabilized ferroelectric liquid crystal will not occur. Secondly, the liquid crystal material used in the above systems can be driven on a low voltage (0 to 5V) and a liquid crystal display apparatus of low power consumption can be attained. Thirdly, display apparatuses of the above systems are highly resistant to the mechanical shock and will not cause destruction of alignment by the mechanical shock which may occur in the surface stabilized ferroelectric liquid crystal.
However, in a case where the active matrix driving operation of liquid crystal having spontaneous polarization is effected, the light transmittance in the ON state is significantly lowered in comparison with the case of static driving operation. As a result, the contrast is lowered and the display quality is degraded.